3.5.4Additional lessons
Epinephrine and Melatonin — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 68 of 142
Part 1 of 4Epinephrine and the Fight or Flight Response
Epinephrine prepares the body for sudden vigorous activity.
Source
- The adrenal medulla secretes epinephrine, also called adrenaline, when sympathetic neurones signal stress, fear or excitement.
Transport
- It is carried in the blood and binds to receptors on target cells in the heart, liver, airways and blood vessels.
Heart
- It acts on the sinoatrial node to increase heart rate, so more blood and oxygen reach the muscles.
Liver
- It causes glycogen to be broken down to glucose, which raises blood glucose for respiration.
Blood flow
- Vessels to skeletal muscle widen while those to the skin and digestive system narrow, which redirects blood to muscles.
Part 2 of 4Melatonin and Circadian Rhythms
Melatonin and Circadian Rhythms
Circadian rhythm
- A daily cycle of about 24 hours in physiology and behaviour, such as sleep, temperature and hormone secretion.
Source
- The pineal gland in the brain secretes melatonin, mostly at night.
Light control
- The retina detects light and dark, and the signal reaches the pineal gland through the hypothalamus, so darkness raises melatonin.
Effect on sleep
- Rising melatonin promotes tiredness and sleep, while falling melatonin at dawn prepares the body to wake.
Free-running cycle
- With no light cues, melatonin still cycles with a period near 24 hours, so light resets the clock each day.
Part 3 of 4Comparing the Two Hormones
Timescale
- Epinephrine acts within seconds during an emergency, whereas melatonin follows a daily rhythm.
Control
- Epinephrine release is triggered by nerves of the sympathetic nervous system, while melatonin release is triggered by light and darkness.
Type of response
- Epinephrine prepares for physical activity and melatonin promotes rest, so they have opposite broad effects.
Disruption
- Shift work or travel across time zones disturbs melatonin timing and causes tiredness while the body clock adjusts.
| Feature | Epinephrine | Melatonin |
|---|---|---|
| Gland | Adrenal medulla | Pineal gland |
| Stimulus | Stress or excitement | Darkness |
| Main effect | Raises heart rate and blood glucose | Promotes sleep |
| Timing | Short-term | Daily cycle |
Part 4 of 4Exam Checklist & Tips
3.5.4 – Key Points for the Exam
- Epinephrine A hormone from the adrenal medulla that prepares the body for fight or flight.
- Melatonin A hormone from the pineal gland that helps regulate the sleep-wake cycle.
- Circadian rhythm A biological cycle with a period of about 24 hours.
- Target cell A cell with specific receptors for a hormone.
Common Mistakes
- Confusing the adrenal medulla with the adrenal cortex or the pituitary gland.
- Saying melatonin is released in response to light; release rises in darkness.
- Stating that epinephrine slows the heart; it increases heart rate.
- Claiming that light is needed for any rhythm; the cycle persists in darkness and light resets it.
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